Tiina Matikainen

Aromatic hydrocarbon receptor-driven Bax gene expression is required for premature ovarian failure caused by biohazardous environmental chemicals

Polycyclic aromatic hydrocarbons (PAHs) are toxic chemicals released into the environment by fossil fuel combustion. Moreover, a primary route of human exposure to PAHs is tobacco smoke. Oocyte destruction and ovarian failure occur in PAH-treated mice, and cigarette smoking causes early menopause in women. In many cells, PAHs activate the aromatic hydrocarbon receptor (Ahr), a member of the Per-Arnt-Sim family of transcription factors. The Ahr is also activated by dioxin, one of the most intensively studied environmental contaminants. Here we show that an exposure of mice to PAHs induces the expression of Bax in oocytes, followed by apoptosis. Ovarian damage caused by PAHs is prevented by Ahr or Bax inactivation. Oocytes microinjected with a Bax promoter–reporter construct show Ahr-dependent transcriptional activation after PAH, but not dioxin, treatment, consistent with findings that dioxin is not cytotoxic to oocytes. This difference in the action of PAHs versus dioxin is conveyed by a single base pair flanking each Ahr response element in the Bax promoter. Oocytes in human ovarian biopsies grafted into immunodeficient mice also accumulate Bax and undergo apoptosis after PAH exposure in vivo. Thus, Ahr-driven Bax transcription is a novel and evolutionarily conserved cell-death signaling pathway responsible for environmental toxicant-induced ovarian failure.

Caspase-3 is a pivotal mediator of apoptosis during regression of the ovarian corpus luteum

Because caspase-3 is considered a primary executioner of apoptosis and has been implicated as a mediator of luteal regression, we hypothesized that corpora lutea (CL) derived from caspase-3 null mice would exhibit a delayed onset of apoptosis during luteal regression, when compared with CL derived from wild-type (WT) mice. To test this hypothesis, ovulation was synchronized in immature (postpartum d 24 –27) WT and caspase-3-deficient female littermates by exogenous gonadotropins. Individual CL were isolated by manual dissection, 30 h after ovulation, and placed in organ culture dishes in the absence of serum and growth factors. At the time of isolation (0 h) and after 24, 48, and 72 h in culture, the CL were removed and assessed for the presence of processed (active) caspase-3 enzyme and for apoptosis by multiple criteria. There was no evidence of active caspase-3 enzyme or apoptosis in either WT or caspase-3-deficient CL before culture. However, CL derived from the WT mice exhibited a time-dependent increase in the level of active caspase-3 and apoptosis during culture. By comparison, CL derived from caspase-3-deficient mice, cultured in parallel, failed to exhibit any detectable active caspase-3 and showed attenuated rates of apoptosis. To extend these findings derived from ex vivo culture experiments, ovaries were collected from WT and caspase-3 null female littermates at 2, 4, or 6 d post ovulation, and the occurrence of apoptosis within the CL was analyzed. Whereas ovaries of WT mice had only residual luteal tissue at d 6 post ovulation, ovaries collected from caspase-3-deficient mice retained many CL, at d 6 post ovulation, that were similar in size to those observed in the early luteal phase of WT mice. Importantly, there was no dramatic increase in apoptosis in CL of caspase-3-deficient mice at any time point examined post ovulation, indicating that the involution process had indeed been delayed. In contrast, the levels of progesterone declined regardless of genotype. These data provide the first direct evidence that caspase-3 is functionally required for apoptosis to proceed normally during luteal regression. However, caspase-3 is not a direct mediator of the decrease in steroidogenesis associated with luteolysis. (Endocrinology 143: 1495–1501, 2002)

Maternal exposure to polycyclic aromatic hydrocarbons diminishes murine ovarian reserve via induction of Harakiri

Maternal smoking during pregnancy is associated with a variety of adverse neonatal outcomes including altered reproductive performance. Herein we provide molecular evidence for a pathway involved in the elimination of the female germline due to prepregnancy and/or lactational exposure to polycyclic aromatic hydrocarbons (PAHs), environmental toxicants found in cigarette smoke. We show that ovaries of offspring born to mice exposed to PAHs contained only a third of the ovarian follicle pool compared with offspring of unexposed female mice. Activation of the cell death pathway in immature follicles of exposed females was mediated by the aryl hydrocarbon receptor (Ahr), as ovarian reserve was fully rescued by maternal cotreatment with the Ahr antagonist, resveratrol, or by inactivation of the Ahr gene. Furthermore, in response to PAHs, Ahr-mediated activation of the harakiri, BCL2 interacting protein (contains only BH3 domain), was necessary for execution of cell death. This pathway appeared to be conserved between mouse and human, as xenotransplanted human ovarian cortex exposed to PAHs responded by activation of the identical cell death cascade. Our data indicate that maternal exposure to PAHs prior to pregnancy and/or during lactation compromises ovarian reserve of female offspring, raising the concern about the transgenerational impact of maternal smoking on ovarian function in the human.

A central role for ceramide in the age-related acceleration of apoptosis in the female germline

An age‐dependent acceleration of apoptosis occurs in female germ cells (oocytes), and this requires communication between the oocyte and its surrounding somatic (cumulus) cells. Here we show in aged mice that ceramide is translocated from cumulus cells into the adjacent oocyte and induces germ cell apoptosis that can be prevented by sphingosine‐1‐phosphate. Trafficking of ceramide requires gap junction‐dependent communication between the cumulus cells and the oocyte as well as intact lipid rafts. Further, the occurrence of the elevated incidence of apoptosis in oocytes of aged females is concomitant with an enhanced sensitivity of the oocyte to a spike in cytosolic ceramide levels, as well as increased bax mRNA and Bax protein levels. Thus, the force driving the age‐related increase in female germ cell death is multifactorial, but changes in the intercellular trafficking of ceramide, along with hypersensitivity of oocytes to ceramide, are key factors in this process.

Caspase-12 compensates for lack of caspase-2 and caspase-3 in female germ cells

Previously, we analyzed mice lacking either caspase-2 or caspase-3 and documented a role for caspase-2 in developmental and chemotherapy-induced apoptosis of oocytes. Those data also revealed dispensability of caspase-3, although we found this caspase critical for ovarian granulosa cell death. Because of the mutual interdependence of germ cells and granulosa cells, herein we generated caspase-2 and -3 double-mutant (DKO) mice to evaluate how these two caspases functionally relate to each other in orchestrating oocyte apoptosis. No difference was observed in the rate of spontaneous oocyte apoptosis between DKO and wildtype (WT) females. In contrast, the oocytes from DKO females were more susceptible to apoptosis induced by DNA damaging agents, compared with oocytes from WT females. This increased sensitivity to death of DKO oocytes appears to be a specific response to DNA damage, and it was associated with a compensatory upregulation of caspase-12. Interestingly, DKO oocytes were more resistant to apoptosis induced by methotrexate (MTX) than WT oocytes. These results revealed that in female germ cells, insults that directly interfere with their metabolic status (e.g. MTX) require caspase-2 and caspase-3 as obligatory executioners of the ensuing cell death cascade. However, when DNA damage is involved, and in the absence of caspase-2 and -3, caspase-12 becomes upregulated and mediates apoptosis in oocytes.

Effects of Acid Sphingomyelinase Deficiency on Male Germ Cell Development and Programmed Cell Death

Abstract Deficiency of acid sphingomyelinase (ASM), an enzyme responsible for producing a pro-apoptotic second messenger ceramide, has previously been shown to promote the survival of fetal mouse oocytes in vivo and to protect oocytes from chemotherapy-induced apoptosis in vitro. Here we investigated the effects of ASM deficiency on testicular germ cell development and on the ability of germ cells to undergo apoptosis. At the age of 20 weeks, ASM knock-out (ASMKO) sperm concentrations were comparable with wild-type (WT) sperm concentrations, whereas sperm motility was seriously affected. ASMKO testes contained significantly elevated levels of sphingomyelin at the age of 8 weeks as detected by high-performance, thin-layer chromatography. Electron microscopy revealed that the testes started to accumulate pathological vesicles in Sertoli cells and in the interstitium at the age of 21 days. Irradiation of WT and ASMKO mice did not elevate intratesticular ceramide levels at 16 h after irradiation. In situ end labeling of apoptotic cells also showed a similar degree of cell death in both groups. After a 21-day recovery period, the numbers of primary spermatocytes and spermatogonia at G2 as well as spermatids were essentially the same in the WT and ASMKO testes, as detected by flow cytometry. In serum-free cultures both ASMKO and WT germ cells showed a significant increase in the level of ceramide, as well as massive apoptosis. In conclusion, ASM is required for maintenance of normal sphingomyelin levels in the testis and for normal sperm motility, but not for testicular ceramide production or for the ability of the germ cells to undergo apoptosis.